1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system for carrying out repeating transmission of a wavelength division multiplexed signal light via optical amplifiers. More particularly, this invention relates to a wavelength division multiplexing optical transmission system in which a hybrid transmission line formed by combining optical fibers having reciprocal wavelength dispersion is adopted for compensating for wavelength dispersion and wavelength dispersion-slope.
2. Description of the Related Art
In the past, with long-distance optical transmission systems, optical reproduction repeaters have been used for transmission. The optical reproduction repeaters convert an optical signal into an electrical signal and perform re-timing, reshaping, and regenerating. However, the employment of optical amplifiers has come to be a matter of common practice these days. An optical amplification repeater transmission mode using optical amplifiers as linear repeaters is under discussion. The optical reproduction repeaters are replaced with optical amplifier repeaters, whereby the number of parts included in a repeater can be reduced greatly. Consequently, high reliability can be guaranteed and a great reduction in cost is expected.
Moreover, a wavelength division multiplexing (WDM) optical transmission mode has attracted people's attention as one of modes capable of realizing a large-capacity optical transmission system. According to the wavelength division multiplexing optical transmission mode, two or more optical signals having different wavelengths are multiplexed and transmitted over one transmission line.
With a WDM optical amplification repeater transmission mode which is a combination of the optical amplification repeater transmission mode and WDM optical transmission mode, wavelength division multiplexed signal lights can be amplified collectively using optical amplifiers. Moreover, large-capacity and long-distance transmission can be achieved with a simple (economic) configuration.
Conventional WDM optical amplification repeater transmission systems (hereafter abbreviated to WDM optical transmission systems) adopt a method of managing wavelength dispersion generated in a transmission line so as to minimize deterioration of transmission characteristics due to a nonlinear effect generated in the transmission line.
For example, in an article (1) written by N. S. Bergano et al. and entitled "Wavelength Division Multiplexing in Long-haul Transmission Systems" (IEEE Journal of Lightwave Technology, Vol. 14, No. 6, PP.1299-1308, 1996), a transmission line made by combining a dispersion-shifted fiber (DSF) and a single-mode fiber (SMF) is used as shown in FIG. 16. The dispersion-shifted fiber has a length of approximately 900 km, and is of a zero-dispersion wavelength .lambda..sub.OD of 1585 nm, and positive wavelength dispersion-slope. The single-mode fiber has a length of approximately 100 km, and is of a zero-dispersion wavelength .lambda..sub.OS of 1310 nm, and positive wavelength dispersion-slope. An average zero-dispersion wavelength .lambda..sub.OA of the transmission line is approximately 1558 nm and wavelengths of signal light permitted to be propagated over the transmission line range are from 1556 nm to 1560 nm.
The value of wavelength dispersion generated in the DSF and SMF are approximately -2 ps/nm/km and approximately +20 ps/nm/km respectively The group velocity of signal light and spontaneously emitted light and the group velocity of each signal light are different between the DSF and SMF The employment of the transmission line made by combining the DSF and SMF makes it possible to shorten the time of interaction by a nonlinear effect. Deterioration in transmission characteristics due to four wave mixing (FWM) and cross phase modulation (XPM) can therefore be minimized. Moreover, since the average zero-dispersion wavelength of the transmission line falls within the wavelengths of signal light, deterioration in transmission characteristics due to self phase modulation (SPM) and wavelength dispersion can also be minimized.
However, a bandwidth used for transmission must be expanded in order to increase a capacity of the WDM optical transmission system. In this case, as far as the foregoing configuration is concerned, due to wavelength dispersion-slope, it is hard to compensate wavelength dispersion so that wavelength dispersion will become zero relative to all wavelengths. Consequently, the waveform of signal light is impaired due to interaction between wavelength dispersion that has not been compensated for but cumulated and the nonlinear effect in an optical fiber.
As a countermeasure, a proposal has been made of a transmission line adopting a dispersion compensation fiber as a second half of a transmission segment thereof. The dispersion compensation fiber compensates for wavelength dispersion and wavelength dispersion-slope generated in a first half of the transmission segment of the transmission line. More particularly, for example, a 1.3 .mu.m zero-dispersion SMF having positive wavelength dispersion and positive wavelength dispersion-slope is used as the first half of the transmission segment of the transmission line. A dispersion compensation fiber having negative wavelength dispersion and negative wavelength dispersion-slope slope so as to compensate for the wavelength dispersion and wavelength dispersion-slope generated in the 1.3 .mu.m zero-dispersion fiber is used as the second half of the transmission segment of the transmission line. Thus, the wavelength dispersion-slope is decreased in order to minimize cumulative wavelength dispersion. Eventually, deterioration in transmission characteristics can be alleviated.
According to an article (2) written by M. Murakami et al. and entitled "Quarter terabit (25.times.10 Gb/s) over 9288 km WDM transmission experiment using nonlinear supported RZ pulse in higher order fiber dispersion managed line" (ECOC'98, PP.79-81, 1998), an average wavelength dispersion-slope may be minimized to 0.0067 ps/nm.sup.2 /km. Specifically, a 1.3 .mu.m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 .mu.m zero-dispersion fiber has a length equivalent to 50% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 50% of the transmission segment and negative wavelength dispersion.
Moreover, according to an article (3) written by K. Yonenaga et al. and entitled "Dispersion-compensation-free 40-Gbit/s.times.4-channel WDM transmission experiment using zero-dispersion-flattened transmission line" (OFC'98, PD20, 1998), an average wavelength dispersion-slope may be minimized to -0.0028 ps/nm.sup.2 /km. Specifically, a 1.3 .mu.m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 .mu.m zero-dispersion fiber has a length equivalent to 55% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 45% of the transmission segment and negative wavelength dispersion.
Furthermore, according to an article (4) written by T. Kashiwada et al. and entitled "Ultra-low chromatic and polarization mode dispersion hybrid fiber links for ultra-high speed transmission systems" (OECC'98, 15C1-3, PP.364-365, 1998), an average wavelength dispersion-slope may be minimized to 0.008 ps/nm.sup.2 /km. Specifically, a 1.3 .mu.m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 .mu.m zero-dispersion fiber has a length equivalent to 84% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 16% of the transmission segment and negative wavelength dispersion.
For realizing a WDM optical transmission system which has a larger capacity and enables longer-distance transmission, a transmission line is requested to meet requirements, namely, (a) a transmission loss must be small, (b) a nonlinear effective area must be large, (c) the wavelength of signal light must not agree with a zero-dispersion wavelength of the transmission line, (d) an averaged value of wavelength dispersion measured in a direction of transmission distance must be negative, (e) a compensation interval of cumulative wavelength dispersion must be sufficiently large relative to an inter-repeater space, and (f) wavelength dispersion-slope must be small or be able to be compensated for.
With a conventional WDM optical transmission system employing a transmission line made by combining a 1.3 .mu.m zero-dispersion fiber having positive wavelength dispersion and a dispersion compensation fiber having negative wavelength dispersion, as described above, the nonlinear effective area of the dispersion compensation fiber used as the second half of the transmission line is relatively small, and a transmission loss thereof is relatively large. Consequently, the system is susceptible to a nonlinear effect and an optical signal-to-noise ratio becomes low. Therefore, even when wavelength dispersion and wavelength dispersion-slope are compensated for, a transmission characteristic is not improved satisfactorily.
Now, the magnitude of improvement in a transmission characteristic will be estimated quantitatively, to clarify problems underlying the WDM optical transmission systems described in the above described articles.
A transmission characteristic of a WDM optical transmission system greatly depends on an optical signal-to-noise ratio. When an output of each repeater is higher and a transmission loss is smaller, the optical signal-to-noise ratio has a larger value. The repeater output and transmission loss can therefore be used as indices indicating the magnitude of improvement in the transmission characteristic of the WDM optical transmission system.
The repeater output is limited by a nonlinear effect generated in a transmission line. It is therefore important to quantitatively estimate the occurrence of the nonlinear effect. In general, the nonlinear effect .phi..sub.NL can be expressed as formula (1) below. ##EQU1##
where .lambda. denotes the wavelength of signal light, n.sub.2 denotes a coefficient of a nonlinear refractive index of a transmission line, and A.sub.eff denotes a nonlinear effective area of the transmission line. Moreover, P denotes optical power, and L denotes a transmission distance.
A 1.3 .mu.m zero-dispersion single-mode fiber (SMF) having positive wavelength dispersion is used as the first half of the transmission line whose whole length is L. A dispersion compensation fiber (reversed dispersion fiber: RDF) having negative wavelength dispersion and negative wavelength dispersion-slope is used as the second half thereof. Herein, a distance from an incident end to a border between the 1.3 .mu.m zero-dispersion SMF and RDF shall be l.sub.b. In this case, the nonlinear effect .phi..sub.NL varies depending on the ratio of the length of the RDF to the length of the transmission segment.
The optical power P(l) at a position separated by a distance .vertline.(0&lt;.vertline.&lt;L) from the incident end of the transmission line is expressed as formula (2) below. EQU P(1)=P(0).multidot.e.sup.-.alpha.1 (2)
where .alpha. denotes a transmission loss (unit: 1/km).
Consequently, the nonlinear effect .phi..sub.NL generated in a hybrid transmission line composed of the 1.3 .mu.m zero-dispersion SMF and RDF can be expressed as formula (3) below based on the formulas (1) and (2). ##EQU2##
The nonlinear effect .phi..sub.NL given by the formula (3) is standardized using, as a reference value, a value of the nonlinear effect when a transmission line is made using only a DSF that is a typical transmission fiber. The nonlinear effective area of the DSF is divided by the standardized nonlinear effect to calculate an average nonlinear effective area in a longitudinal direction of the hybrid transmission line with respect to the nonlinear effective area of the transmission line formed with the DSF alone. Consequently, the magnitude of relaxation in the nonlinear effect is calculated based on the nonlinear effect of the transmission line formed with the DSF alone, that is, the magnitude of relaxation in the upper limit of repeater output, is calculated.
The magnitude of improvement in a transmission characteristic of the hybrid transmission line with respect to the transmission characteristic of the transmission line formed with the DSF alone, resulting from relaxation in the upper limit of repeater outputs is expressed as follows: EQU 10 Log{A.sub.eff (1 )/A.sub.eff (2)} (dB)
where A.sub.eff (1) denotes the average nonlinear effective area in the longitudinal direction of the hybrid transmission line, and A.sub.eff (2) denotes the nonlinear effective area of the transmission line formed with the DSF alone.
The magnitude of improvement in a transmission characteristic resulting from reduction in a transmission loss is expressed as follows: EQU {Loss(2)-Loss(1)}.times.Length of transmission segment (dB)
where Loss(1) denotes a value (expressed in the unit of dB) indicating an average transmission loss in the longitudinal direction of the hybrid transmission line, and Loss(2) denotes a value (in the unit of dB) indicating an average transmission loss in a longitudinal direction of the transmission line formed with the DSF alone.
The magnitude of improvement I in a transmission characteristic of a system using the hybrid transmission line relative to a transmission characteristic of a system using the transmission line formed with the DSF alone can be evaluated quantitatively according to formula (4) below. EQU 1=10 Log{A.sub.eff (1)/A.sub.eff (2)}-{Loss(2)-Loss(1)} (dB)
For example, assume that the transmission distance L is 50 km. The formula (4) expressing the magnitude of improvement I is solved using values listed in Table 1 of parameters indicating the characteristics of the 1.3 .mu.m zero-dispersion SMF, RDF, and DSF respectively.
TABLE 1 Parameters of 1.3 .mu.m zero- characteristics dispersion SMF RDF DSF Nonlinear effective area 80 20-40 50 A.sub.eff (.mu.m.sup.2) Coefficient of nonlinear 2.8 .times. 10.sup.-20 3.6 .times. 10.sup.-20 3.3 .times. 10.sup.-20 refractive index n(m.sup.2 /W) Transmission loss Loss 0.18 0.2-0.5 0.2 (dB/km)
The respective parameter values concerning the 1.3 .mu.m zero-dispersion SMF in Table 1 are referred to the above article (4). The respective parameter values concerning the RDF are referred to the above article (2) and an article (5) written by M. Onishi et al. and entitled "Optimization of dispersion-compensating fibers considering self-phase modulation suppression" (OFC'96, ThA2, PP.200-201, 1996). The respective parameter values concerning the DSF are referred to the above article (5). Moreover, in Table 1, the transmission loss is expressed in the unit of dB/km. For solving the formula (3), a value converted to the unit of 1/km is used as .alpha..
FIG. 17 is a graph showing the values of the magnitude of improvement I in a transmission characteristic which are calculated in accordance with the foregoing conditions. The lengths of the SMF and RDF on the axis of abscissas indicate the length of the 1.3 .mu.m zero-dispersion SMF used as the first half of the transmission segment and the length of the RDF used as the second half thereof, respectively. The axis of ordinates indicates the magnitude of improvement I in a transmission characteristic of a hybrid transmission line. The values of the magnitude of improvement I are plotted according to the ratio of the length of the RDF within the transmission segment corresponding to the combination of the respective parameters concerning the RDF (transmission loss Loss and nonlinear effective area A.sub.eff).
As shown in FIG. 17, compared with when the transmission line is formed with the DSF alone, a transmission characteristic of the hybrid transmission line tends to be improved as the length of the RDF serving as the second half gets shorter.
As for the WDM optical transmission systems described in the foregoing articles (2) and (3), the magnitude of improvement in a transmission characteristic will be discussed using the foregoing method. However, the articles do not refer to the practical characteristics of a dispersion compensation fiber (RDF) used as the second half of the transmission segment of the transmission line. For the characteristics, an article (6) written by K. Mukasa et al and entitled "Novel network fiber to manage dispersion at 1.55 .mu.m with combination of 1.3 .mu.m zero dispersion single mode fiber" (ECOC'97, PP. 127-130, 1997) was referenced.
In the WDM optical transmission systems described in the articles (2) and (3), the ratio of the length of the RDF, of which nonlinear effective area is relatively small, to the length of the transmission segment is large (about 50%). An average nonlinear effective area within the transmission segment is therefore small. This hinders alleviation of a nonlinear effect.
Specifically, the nonlinear effective area of the 1.3 .mu.m zero-dispersion SMF used as the first half of the transmission line is as large as approximately 80 .mu.m.sup.2. A transmission loss occurred in the 1.3 .mu.m zero-dispersion SMF is as small as approximately 0.20 dB/km. The diameter of a mode field in the RDF used as the second half of the transmission line is 5.8 .mu.m. In other words, the nonlinear effective area of the RDF is as small as approximately 26 .mu.m.sup.2, and a transmission loss occurred in the RDF is as large as approximately 0.25 dB/km. The nonlinear effect .phi..sub.NL in the transmission line is calculated according to the formula (3), and then an average nonlinear effective area in the longitudinal direction of the transmission line with respect to the nonlinear effective area of the transmission line formed with the DSF alone is calculated to be approximately 49 .mu.m.sup.2. Moreover, a transmission loss occurred in the transmission line is approximately 0.225 dB/km. In this state, the magnitude of improvement I in a transmission characteristic is calculated according to the formula (4). The results of calculation are plotted as white star mark in FIG. 17.
Now, consideration is taken into the fact that the nonlinear effective area of a DSF generally employed as a transmission line is approximately 50 .mu.m.sup.2 and a transmission loss occurred in the DSF is 0.20 dB/km. According to the configurations of the WDM optical transmission systems described in the articles (2) and (3), wavelength dispersion and wavelength dispersion-slope can be compensated for. However, the results of improving a nonlinear effective area and transmission loss are limited. Compared with the transmission characteristic of the transmission line formed with the DSF alone, the transmission characteristic of the hybrid transmission line is thought to be poorer by approximately 0.85 dB.
The problems underlying the WDM optical transmission system described in the foregoing article (4) will be discussed below.
The RDF referred to in the article (4) is designed to compensate for wavelength dispersion accumulated by the first-half 1.3 .mu.m zero-dispersion SMF in a state where the length of transmission line is relatively short. The magnitude of compensation for wavelength dispersion per distance must therefore be relatively large. This results in a large transmission loss and a small nonlinear effective area. The thesis (4) does not refer to the nonlinear effective area of the RDF. The nonlinear effective area of an RDF described in the article (6) will therefore be used as a reference, because the wavelength dispersion per distance in this RDF is equivalent to approximately -100 .mu.s/nm/km.
The nonlinear effective area of the first-half 1.3 .mu.m zero-dispersion SMF is as large as approximately 80 .mu.m.sup.2. A transmission loss occurred in the SMF is as small as approximately 0.20 dB/km. By contrast, the nonlinear effective area of the second-half RDF is as small as approximately 20 .mu.m.sup.2, and a transmission loss occurred in the RDF is as large as approximately 0.5 dB/km. A nonlinear effect .phi..sub.NL of the transmission line is calculated according to the formula (3), and then an average nonlinear effective area in the longitudinal direction of the transmission line formed with the DSF alone is calculated to be approximately 68 m.sup.2. Moreover, a transmission loss occurred in the transmission line is approximately 0.23 dB/km. The magnitude of improvement I in a transmission characteristic is calculated according to the formula (4). The results of calculation are plotted as black star mark in FIG. 17. Consideration is taken into the nonlinear effective area of the DSF and a transmission loss occurred in the DSF. Even in the configuration of the WDM optical transmission system described in the article (4), the results of improving the nonlinear effective area and transmission loss are limited. Compared with the transmission characteristic of the transmission line formed with the DSF alone, the transmission characteristic of the hybrid transmission line is thought to be poorer by approximately 0.19 dB.
As mentioned above, as far as the conventional WDM optical transmission systems described in the articles are concerned, although wavelength dispersion and wavelength dispersion-slope may be compensated for, the result of improving the transmission characteristic is unsatisfactory.
Moreover, a WDM optical transmission system may be configured in such a manner that repeaters are linked using a hybrid transmission line made by combining a 1.3 .mu.m zero-dispersion SMF and an RDF. In this case, when a cumulative value of wavelength dispersion (cumulative wavelength dispersion) generated in each inter-repeater segment is positive, the peak value of optical power becomes large due to compressed optical pulses, to thereby become more susceptible to a nonlinear effect.
The foregoing problems will be described by taking the WDM optical transmission system described in the article (2) for instance.
According to the system configuration described in the article (2), a transmission line made by combining a 1.3 .mu.m zero-dispersion SMF and an RDF is laid down as each inter-repeater segment. A transmission line formed with a dispersion compensation fiber for compensating for cumulative wavelength dispersion generated in a plurality of inter-repeater segments is laid down at intervals of a given number of inter-repeater segments.
FIGS. 18A and 18B graphically show an example of wavelength dispersion map created using the parameters mentioned in the article (2). FIG. 18A shows a change in wavelength dispersion over ten inter-repeater segments. FIG. 18B shows a change in wavelength dispersion over one hundred inter-repeater segments.
In the example shown in FIGS. 18A and 18B, the length of one inter-repeater segment is 50 km, and cumulative wavelength dispersion is compensated for at intervals of five inter-repeater segments. A compensation interval of cumulative wavelength dispersion is five times as large as the inter-repeater segment. An averaged wavelength dispersion in the transmission distance direction is -225 ps/nm/km. The value of cumulative wavelength dispersion becomes positive in first, second, sixth, and seventh inter-repeater segments. The peak value of optical power gets larger due to effect of optical pulse compression, to thereby become more susceptible to a nonlinear effect. Moreover, since the compensation interval of cumulative wavelength dispersion is relatively short, the value of cumulative wavelength dispersion is frequently reset to zero. There arises a possibility that the waveform is distorted due to the nonlinear effect.
FIGS. 19A and 19B show an example of wavelength dispersion map, wherein a compensation interval of cumulative wavelength dispersion is ten times as large as an inter-repeater segment. An averaged wavelength dispersion in the transmission distance direction is -225 ps/nm/km. Even in this case, the value of cumulative wavelength dispersion becomes positive in the first to fourth inter-repeater segments. The peak value of optical power increases due to effect of optical pulse compression, to thereby become susceptible to a nonlinear effect. In a region where the value of cumulative wavelength dispersion is positive, WDM signal light is propagated over successive segments, to thereby be more susceptible to a larger nonlinear effect due to the optical pulse compression.
Incidentally, it is pointed out by H. Taga et al. in their article (7) entitled "Performance Evaluation of the Different Types of Fiber-Wavelength dispersion Equalization for IM-DD Ultralong-Distance Optical Communication Systems with Er-Doped Fiber Amplifiers" (IEEE Journal of Lightwave Technology, Vol. 12, No. 9, September, 1994) that a transmission characteristic gets worse with an increase in the number of opportunities on which light is propagated over a transmission line in a region where the value of cumulative wavelength dispersion is positive.